#include<iostream>
#include<fstream>
#include<sstream>
#include<string>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<ctime>
#include<stack>
#include<queue>
#include<set>
#include<map>
#include<vector>
#include<list>
#include<algorithm>
#include<utility>
#include<complex>
#include<functional>
using namespace std;

bool used[100];
int p[100];
int num = 0;
vector<list<int>> good;
vector<list<int>> bad;
int level[100];
void dfs(int s){
	used[s] = true;
	for(auto v :good[s])
	{
		if (used[v])continue;
		dfs(v);
	}
	p[num++] = s;
}

int bfs(int s){

	int res = level[s];
	int mini = res;
	int cnt = 1;
	queue<int> que;
	que.push(s);
	used[s] = true;
	if (bad[s].size() == 0)return level[s] * 2;
	while (que.empty() == false){
		auto v = que.front(); que.pop();
		//cout << v << " ";
		for(auto u: bad[v])
		{
			if (used[u])continue;
			used[u] = true;
			que.push(u);
			res += level[u];
			mini = min(mini, level[u]);
			cnt++;
		}


	}
	if (cnt > 1)res += mini;
//	cout << endl;
	return res;
}
int main(void){
	int N,s,sum=0;
	cin >> N;
	good.resize(N);
	bad.resize(N);
	for (int i = 0; i < N; i++)
	{
		used[i] = false;
		cin >> level[i] >> s;
		s--;
		if (s != i){
			good[s].push_back(i);
			bad[i].push_back(s);
		}
	}
	for (int i = 0; i < N; i++)
	{
		if (used[i])continue;
		dfs(i);
	}
	reverse(p, p + N);
	for (int i = 0; i < N; i++)
	{
		used[i] = false;
	//	cout << p[i] << " ";
	}
	//cout << endl;
	for (int i = 0; i < N; i++)
	{
		if (used[p[i]])continue;
		sum += bfs(p[i]);
	}
	cout << sum / 2 ;
	if (sum & 1)cout << ".5";
	else cout << ".0";
	cout << endl;
	return 0;
}